The invention pertains to the thermo-oxidative stabilization of black pigmented polyolefins, e.g. polyethylene and polypropylene.
Compounded polyolefin resins, including polyethylene and polypropylene are increasingly sought for articles which can withstand long term high temperature service, such as automotive under-the-hood parts. The physical, chemical and electrical properties of polyolefin require the use conventionally of carbon black which can improve their stability against light or for coloring, but have a known deleterious effect on long term heat aging performance. Also, other inorganic fillers are sometimes incorporated therein to improve the mechanical properties of polyolefins. On the other hand, polyolefin resins are highly susceptible to thermal oxidative deterioration, and therefore in the past methods phenolic antioxidants and sulfur antioxidants have been used in combination for their antioxidation. Nevertheless, particularly polyolefin resin compositions containing carbon black with these stabilizers still have a disadvantage of less than desired stability against thermo-oxidation, and thus their improvement is sought.
The shortcoming in thermo-oxidative stability of polyolefins containing carbon black and/or inorganic fillers is reported to be due in large part to deactivation of stabilizers by carbon black and inorganic fillers. Adsorption of stabilizers onto the surface of carbon black particles, and absorption into the interstices of the carbon black particles has been previously shown.
In order to improve the stability of polyolefin resins containing carbon black against thermo-oxidation, some proposals have been made, including a method in which a metal deactivator is incorporated therein (Japanese Patent Kokai No. 207733/91), a method in which a piperidine compound is incorporated therein (Japanese Patent Kokai No. 42441185 and Plastics Compounding, 1987, July/August, pp. 35, 38, 39), and the incorporation of epoxy resins which provide an impervious surface layer on carbon black.
U.S. Pat. No. 5,214,084 discloses an inorganic filled polypropylene stabilized by incorporating therein the following components:
a hindered phenolic compound having a molecular weight of not less than 500,
a high-molecular hindered piperidine compound,
a low-molecular hindered piperidine compound,
a benzoate compound such as 2,4-di-t-butylphenyl 3,5-di-t-butyl-4-hydroxybenzoate,
a phosphorous compound, and
an amide compound such as ethylene-bis stearylamide.
Furthermore, a method is proposed in U.S. Pat. No. 4,985,480 wherein an amide compound is added to polyolefin resins which contain an inorganic filler such as talc, to improve their stability against thermal oxidation.
U.S. Pat. No. 4,925,889 discloses a carbon black filled polyolefin, a stabilizing amount of a N-phenyl-Nxe2x80x2-(p-toluenesulfonyl)-p-phenylenediamine first stabilizer and a second stabilizer component of at least one amine antioxidant selected from the group consisting of a para-substituted, aralkyl-substituted diphenylamine; a para-phenylenediamine and a polymerized dihydroquinoline incorporated into said polyolefin.
A number of substituted phenol compounds, e.g. trialkylphenols, for example 2,6-di-tert-butyl-4-methylphenol and 2,4-dimethyl-6-(1-methylpentadecyl)phenol (U.S. Pat. No. 5,098,945), and their use for stabilizing organic materials are known, and the stabilizing action of sterically hindered phenyls is also described, inter alia, in U.S. Pat. No. 3,944,594, U.S. Pat. No. 3,644,482, U.S. Pat. No. 5,086,173, U.S. Pat. No. 3,644,482, CA 843 985, U.S. Pat. No. 3,681,417.
In general, substituted phenyl compounds that exhibit antioxidant properties for thermoplastics in general contain a phenol group which is substituted by 1 to 3 radicals such as o-, m- or p-methylphenyl, 2,3-dimethylphenyl, 2,4-dimethylphenyl, 2,5-dimethylphenyl, 2,6-dimethylphenyl, 3,4-dimethylphenyl, 3,5-dimethylphenyl, 2-methyl-6-ethylphenyl, 2-methyl-4-tert-butylphenyl, 2-ethylphenyl, 2,6-diethylphenyl, 2,6-diethyl-4-methylphenyl, 2,6-diisopropylphenyl, 4-tert-butylphenyl, p-nonylphenyl, o-, m- or p-chlorophenyl, 2,3-dichlorophenyl, 2,4-dichlorophenyl, 2,5-dichlorophenyl, 2,6-dichlorophenyl, 3,4-dichlorophenyl, 2,4,5-trichlorophenyl, 2,4,6-trichlorophenyl, o-, m- or p-hydroxyphenyl, o-, m- or p-methoxyphenyl, o- or p-ethoxyphenyl, 2,4-dimethoxyphenyl, 2,5-dimethoxyphenyl, 2,5-diethoxyphenyl, o-, m- or p-methoxycarbonyl, 2-chloro-6-methylphenyl, 3-chloro-2-methylphenyl, 3-chloro-4-methylphenyl, 4-chloro-2-methylphenyl, 5-chloro-2-methylphenyl, 2,6-dichloro-3-methylphenyl, 2-hydroxy-4-methylphenyl, 3-hydroxy-4-methylphenyl, 2-methoxy-5-methylphenyl, 4-methoxy-2-methylphenyl, 3-chloro4-methoxyphenyl, 3-chloro-6-methoxyphenyl, 3-chloro-4,6-dimethoxyphenyl and 4-chloro-2,5-dimethoxyphenyl groups.
U.S. Pat. No. 5,376,290 discloses substituted bis-(mercaptomethyl)-phenols as stabilizers, for example: 2,6-bis-(2xe2x80x2-hydroxyethylthiomethyl)-4-methylphenol, 2,6-bis-(2xe2x80x2,3xe2x80x2-dihydroxypropylthiomethyl)-4-methylphenol, 2,6-bis-(2xe2x80x2-methylaminocarbonylethylthiomethyl)-4-phenylphenol, 2,6-bis-(N,N-diethylaminocarbonyl-ethylthiomethyl)-4-allyl-phenol, 2,6-bis-(n-octylthiomethyl)-4-methylphenol, 2,6-bis-(t-octylthiomethyl)-4-t-butyl-phenol.sup.1,2,6-bis-(t-dodecylthiomethyl-4-t-octyl-phenol.sup.2,2,6-bis-(benzylthiomethyl)-6-methylphenol, 2,6-bis-(phenylthiomethyl)-4-t-butyl-phenol, 2,6-bis-(2xe2x80x2-ethylhexyloxycarbonylmethylthiomethyl)-4-cyclohexyl-phenol, 2,6-bis-(n-octadecyloxycarbonylmethylthiomethyl)-4-propargyl-phenol, 2,6-bis-[2xe2x80x2-(2xe2x80x3-ethylhexyloxycarbonyl)-ethylthiomethyl]4-t-butylphenol, 2,2-bis-[4xe2x80x2,4xe2x80x3-dihydroxy-3xe2x80x2,3xe2x80x3,5xe2x80x2,5xe2x80x3-tetrakis-(octylthiomethyl)-phenol]-propane, 2,2-bis-]4xe2x80x2,4xe2x80x3-dihydroxy-3xe2x80x2,3xe2x80x3,5xe2x80x2,5xe2x80x3-tetrakis-(dodecylthiomethyl)-phenyl]- methane and bis-[4xe2x80x2,4xe2x80x3-dihydroxy-3xe2x80x2,3xe2x80x3,5xe2x80x2,5xe2x80x3-tetrakis-(2-ethylhexyloxycarbonylmethylth iomethyl)-phenyl] thioether.
Other known primary antioxidant include tetrakis[methylene-3-(3xe2x80x2,5xe2x80x2-di-tert-butyl-4xe2x80x2-hydroxyphenyl)propionate]methane, sold under the IRGANOX tradename.
In xe2x80x9cAtmospheric Oxidation and Antioxidantsxe2x80x9d; Elsevier Publishing Company (1965), pages 120-125, the author describes the connection between stabilizing action and substitution at the phenol for mineral oils.
U.S. Pat. No. 3,344,113 discloses the use of a mixed esters of thiopropionic acid, e.g., dilauryl, distearyl, and lauryl stearyl thiodipropionates as an improved system relative to the individual stabilizers in polyolefins.
U.S. Pat. No. 3,511,802 discloses the stabilization of polypropylene resins with alkyl-substituted phenols. The preparation as well as the spectra of secondary alkylphenols are given in Chemical Abstracts 69:10147s and 72:11860t.
xe2x80x9cRubber Chemistry and Technologyxe2x80x9d 47 (1974), No. 4, pages 988 and 989, describes the mode of action of antioxidants.
Elemental sulfur compounds and diaryl disulfides are reported as effective hydroperoxide decomposers by generating sulfur dioxide. Synergistic improvement in combinations with alkylated phenol anitoxidant is also reported. Hawkins and Sutter, J of Polymer Sci. 1, pp. 3499-3509 (1963).
C. Uzelmeier, of Shell Chemical, has reported the mechanism by which pigments can degrade the heat resistance of polypropylene. Interaction between carbon black and stabilizers by way of absorption and reaction with the carbon black particle surface are well documented. W/L. Hawkins, et al, J Appl. Polym. Sci., 1,37 (1959). Carbon black alone is seen to extend the oxygen induction period of polypropylene at 150xc2x0 C. at 1 atmosphere oxygen. Carbon black accelerates autoxidation from heat by 56% at levels of 0.1 phr relative to unpigmented, heat-stabilized resin. None of the pigments reported improves the oven heat-aging of polypropylene.
GB 820,207 discloses the use of water soluble disperse dyes extended with vinylic fillers. Vinylic fillers are polymerized, crosslinked, rigid particles containing organic surfaces. These particles are treated with a water solution of the leuco derivative of dyestuffs, followed by insolubilization and deposition of the dye, such as by oxidation, with adsorption onto the particle surface. The cationized pigments can be used in a variety of applications, such as for printing inks or plastics. One drawback to the use of cationic dyes is the obvious additional cost and complexity of reacting the dyestuffs with metal cations, or cationic polymers, as well as the toxicological, and prodegradative effects these cationic materials themselves can have on polymers.
U.S. Pat. No. 5,326,622 discloses an inked heat transferable ribbon containing an ink layer providing a colorant, a binder, and a pressure sensitive adhesive. The colorant located in the ink layer is dispersed in a binder of a wax and a tackifying resin, and is selected from the group consisting of carbon black, iron oxide, white lead, red lead, chrome yellow, vermilion, ultramarine blue, iron blur, cobalt oxide, strontium chromate, titanium yellow, black-titanium oxide, black iron oxide, molybdenum white, lithopone, cobalt blue, azo, phthalocyanine blue, lake, isoindolinone, quinacridone, dioxazineviolet, perinone, perylene, disperse dyes, cationic dyes, basic dyes, acid dyes, metal complex dyes, reactive dyes, direct dyes, sulphur dyes, sulphur vat dyes, vat dyes, azoic dyes, solvent dyes and pigment resin colors. The control layer, above the ink layer contains thermoplastic resin and a tackifying resin. This layer has a higher viscosity, heat sensitive adhesiveness, and hardness than said ink layer. Thermoplastic resin used in the control layer is chosen from ethylene-vinyl acetate copolymer, poly(vinylacetate), ionomer, acrylic polymer, ethylene-ethyl acrylate copolymer, ethylene-ethyl acrylate copolymer, vinyl chloride-vinyl acetate copolymer, poly(vinyl butyral), poly(vinylpyrrolidone), poly(vinyl alcohol), polyamide, ethyl cellulose and polyolefin.
GB application no. 9517103.9 discloses the use of micron-sized, oxidized sulfur pigments as antioxidant in polymers, e.g., polyolefins. Further stabilizers suggests as adjuncts with sulfur dyes include benzofuran. The working examples illustrate that a masterbatch of sulfur brown 96 which is incorporated into polypropylene (PP), and extruded five times, shows a comparable increase in melt viscosity as compared to unstabilized PP and PP containing a well known phenolic antixodant. No long-term heat aging data was provided.
It has been found that fine particle-sized sulfur black in its oxidized state provides no inherent antioxidative effect alone in polyolefins. A surprising synergistic improvement in long term heat aging of stabilized polyolefin is observed, as measured by oxygen induction time and oven heat aging when non-cationized, nonsilylated sulfur black is combined with a phenolic antioxidant and sulfur-containing secondary antioxidant, in comparison to carbon black together with the same stabilizer package. Furthemore, when non-cationized, non-silylated sulfur black is treated by washing and reducing the level of sodium salts, together with a lowering of pH to below 7, this treatment provided further unexpected improvements in the long term heat aging of polyolefins.
Polyolefin compounds and articles therefrom according to the invention comprise, in a conventional solid, melt-phase compounding, and in the absence of carbon black, containing oxidized, non-cationized, non-silylated sulfur black pigment, a phenolic antioxidant and a sulfur-containing secondary stabililzer, each in stabilizing amount, specified below, which provide a synergistic improvement in the long term heat aging of polyolefins.
In another aspect the invention comprises the melt-phase compounding of polyolefins and molded articles made therefrom comprising incorporating of carbon black, oxidized, non-cationized sulfur black pigment, a phenolic antioxidant and sulfur-containing secondary stabilizer.
In a third aspect, there is provided a melt-phase mixed polyolefin compound comprising resin, washed, non-cationized, oxidized sulfur black, together with a phenolic antioxidant and sulfur-containing compound.